Experiencing discomfort as a side effect of using VR systems has been the largest threat to widespread adoption of the technology over the past decades. It is considered the main reason for its failure to live up to overblown expectations in the early 1990s. Few people want a technology that causes them to suffer while using it, and in many cases long after using it. It has also been frustrating for researchers to characterize VR sickness because of many factors such as variation among people, adaptation over repeated use, difficulty of measuring symptoms, rapidly changing technology, and content-dependent sensitivity. Advances in display, sensing, and computing technologies have caused the adverse side effects due to hardware to reduce; however, they nevertheless remain today in consumer VR headsets. As hardware-based side effects reduce, the burden has been shifting more toward software engineers and content developers. This is occurring because the VR experience itself has the opportunity to make people sick, even though the hardware may be deemed to be perfectly comfortable. In fact, the best VR headset available may enable developers to make people more sick than ever before! For these reasons, it is critical for engineers and developers of VR systems to understand these unfortunate side effects so that they determine how to reduce or eliminate them for the vast majority of users.